1. Field of the Invention
The present invention relates to a glass composition that can function as a light emitter or an optical amplification medium in the infrared region that is used in optical communications.
2. Description of the Related Art
Glass or glass-ceramic materials that include ions of a rare earth element such as neodymium (Nd), erbium (Er), praseodymium (Pr) and emit fluorescence in the infrared region have been known. Laser emission and optical amplification that were achieved using these materials were studied, mainly in the 1990s, and they are used as glass lasers or erbium doped optical fiber amplifiers in high-output laser applications and optical communications.
Fluorescence from the materials is caused by radiative transition of the 4f electron of a rare earth ion. Since the 4f electron is covered with an outer-shell electron, the fluorescence can be obtained only in a narrow wavelength region. This limits the ranges of the wavelengths of light that can be amplified and the wavelengths at which laser oscillation can occur, and thus the available wavelength range is narrow.
With consideration given to this, each of JP11(1999)-317561A and JP2001-213636A discloses an optical amplification glass. The glass composition includes a large amount (for instance, at least 20 mol %) of Bi2O3 as well as Er as a fluorescent element and that allows a wavelength range of 80 nm or longer to be used.
JP6(1994)-296058A, JP2000-53442A, and JP2000-302477A each disclose a glass composition for optical amplification. The glass composition includes Cr or Ni as a fluorescent element instead of rare earth elements and allows fluorescence to occur in a wide wavelength range.
JP11(1999)-29334A discloses a Bi doped silica glass. In this glass composition, Bi has been clustered in zeolite and thereby fluorescence is obtained over a wide wavelength range.
The glass composition disclosed in JP11(1999)-317561A and JP2001-213636A includes a large amount of Bi2O3 and has a high refractive index, and thus a wavelength range in which the fluorescence is obtained or light can be amplified is enlarged. However, since the fluorescent source is Er, the extension of the wavelength range is limited to about 100 nm. In addition, the refractive index of the glass composition is as high as about 2. Accordingly, when it is connected to a silica glass optical fiber that is generally used in optical communications, a problem tends to occur that is caused by reflection at the interface therebetween.
The glass composition disclosed in JP6(1994)-296058A contains Al2O3 as a main component as well as Cr, and does not contain a glass network former or contains only a small amount (at most 20 mol %) of a glass network former. Therefore, the glass composition does not have a sufficient glass forming ability and tends to devitrify when being melted or formed.
The glass composition disclosed in JP2000-53442A and JP2000-302477A includes at least one of Ni+ ions, microcrystals including Ni2+ ions and nickel ions having a hexacoordinate structure, and fine particles of metal Ni tend to deposit. Accordingly, this glass composition also tends to devitrify.
In the silica glass disclosed in JP 11(1999)-29334 A, Bi has been clustered and therefore respective Bi elements are extremely close to each other. Hence, deactivation tends to occur between adjacent Bi elements, which results in lower efficiency in optical amplification. Since this silica glass is produced using a sol-gel method, the occurrences of shrinkage during drying and cracks during baking are problems in mass production of large-sized glass or optical fibers.
Accordingly, in the wavelength range for optical communications, only the wavelength range in which a rare earth element such as Nd, Er and Pr emits fluorescence can be used for optical amplification. The other wide wavelength range is difficult to use for optical communications, since transmission loss cannot be compensated by optical amplification.